The present invention relates to a method and apparatus for diagnosing an engine exhaust gas purification system in which an air-fuel ratio sensor or oxygen sensor (hereinafter referred to as the air-fuel ratio sensor or O.sub.2 -sensor representatively) and a catalytic converter are employed.
The system for purifying the exhaust gas of an internal combustion engine (hereinafter also referred to simply as the engine) is primarily comprised of a catalytic converter and an air-fuel ratio feedback apparatus. The catalytic converter is disposed in an exhaust pipe for eliminating HC (hydro-carbon), NO.sub.x (oxides of nitrogen) and CO (carbon monoxide) contained in the exhaust gas discharged from the engine. On the other hand, the air-fuel ratio feedback control apparatus is installed for controlling the amount of fuel injected to the engine by making use of an output signal of an oxygen sensor disposed in the exhaust pipe upstream of the catalytic converter for thereby maintaining the air-fuel ratio essentially at a constant level because the air-fuel ratio has to be held constant in order that the catalytic converter works most efficiently. In the case of a three-way catalyst system, degradation in the performance in the oxygen sensor disposed upstream of the catalytic converter causes the air-fuel ratio to deviate from a narrow range predetermined around a center value given by the stoichiometric air-fuel ratio, as a result of which the conversion efficiency of the catalytic converter for converting the HC, CO and NOx components of the exhaust gas to harmless components undergoes degradation. Besides, deterioration of the performance of the catalytic converter itself will be accompanied with degradation in the efficiency of conversion even when the air-fuel ratio is controlled accurately. As a technique for diagnosing the catalytic converter as to whether it suffers deterioration in the conversion performance or capability, there may be mentioned a diagnosis system disclosed, for example, in JP-A-5-171924 and the corresponding U.S. Pat. No. 5,341,642. More specifically, this publication disclose an apparatus for diagnosing an engine exhaust gas purifying system, which is comprised of an upstream air-fuel ratio sensor for detecting an air-fuel ratio at a location upstream of a catalytic converter as viewed in the exhaust gas flowing direction, a downstream air-fuel ratio sensor for detecting an air-fuel ratio within the exhaust pipe at a location downstream of the catalytic converter, a characteristic waveform extracting means for attenuating signal components of a frequency band which is lower than a air-fuel ratio control signal frequency band of an air-fuel ratio control means in the output signals of the upstream air-fuel ratio sensor and the downstream air-fuel ratio sensor, a means for arithmetically determining a correlation function of the signal passed through the characteristic waveform extracting means, and a catalyst state decision means for deciding the state of deterioration of the catalytic converter on the basis of values of the correlation function.
The performance of the catalytic converter is susceptible to the influence exerted by the temperature of the catalyst. When this temperature is low, the conversion efficiency becomes lowered. Accordingly, unless the temperature of the catalyst is taken into consideration in diagnosing the performance of the catalytic converter, an erroneous or false diagnosis will undesirably be resulted because lowering of the conversion efficiency of the catalytic converter under the influence of a change in the temperature thereof which is affected by the engine speed (rpm) and engine operation state such as an engine load level may erroneously be decided as deterioration in the performance of the catalytic converter. For solving this problem, it is required to install a temperature sensor for correcting the result of the diagnosis, as is disclosed in JP-A-5-171924 and corresponding U.S. Pat. No. 5,341,642. However, the catalyst temperature sensor mentioned above has to be implemented so as to cover a very wide range of temperatures for measurement on the order of 0.degree. C. to 800.degree. C., which involves high expensiveness in manufacturing the diagnosis apparatus, giving rise to another problem.
The problem mentioned just above may be solved by estimating the temperature of the catalytic converter on the basis of a signal measured for other purpose in place of the installing the catalyst temperature sensor. In this conjunction, it should however be mentioned that the means for allowing the catalyst temperature to be measured with high or satisfactory accuracy required for diagnosing the catalytic converter as to deterioration in the performance thereof has been neither proposed nor realized heretofore.